Fast neutron nuclear reactors generally comprise a main vessel of large dimensions, filled with liquid sodium surmounted by an inert gas such as argon, and closed by a very thick, horizontal slab in which openings are provided for the reactor components to pass through.
In particular, the primary pumps which circulate the liquid sodium filling the vessel, and the intermediate exchangers which effect the heat exchange between the primary sodium filling the vessel and the secondary sodium carrying the heat to the steam generators, pass through the slab inside openings of large dimensions and rest on the reactor slab via a flange fixed to their upper parts.
The intermediate heat exchangers comprise a lower part of large diameter, immersed in the liquid sodium, so that the passage in the slab must have a sufficiently large diameter to allow the intermediate exchanger to be introduced or removed without difficulty.
A peripheral space of sufficiently large width is therefore left around the upper part of the intermediate exchanger, in the opening provided in the reactor slab. Hence, it is necessary to limit the circulation of the hot gases and to prevent the escape of the radiation originating from the irradiated elements of the reactor in this peripheral space.
Moreover, the secondary sodium circulates firstly from top to bottom through a passage located in the central part of the exchanger, and then from bottom to top through a nest of tubes where the heat exchange with the primary fluid takes place.
On leaving the nest of tubes, the hot secondary liquid sodium circulates from bottom to top through an annular passage limited in its central part by the sodium downflow passage, and in its peripheral part by a sleeve with a vertical axis, constituting the envelope of the intermediate exchanger.
In the upper part of the exchanger, this sleeve divides into an inner sleeve, inside which the hot secondary sodium circulates, and an outer sleeve, to which the flange of the exchanger is fixed. The inner sleeve and outer sleeve are joined to one another and to the upper part of the exchanger by a symmetrical connector with a Y-shaped cross-section, which is a body of revolution.
It is important to protect the passage through the slab, in particular in its upper part, against the heat evolved by the secondary liquid sodium circulating in the intermediate exchanger.
It is also important to protect the connector between the sleeves of the intermediate exchanger against unfavorable heat conditions, this connector being extremely important for the mechanical strength of the intermediate exchanger.